Pressure transducer

ABSTRACT

A pressure transducer employs a transducer element which is supported on the transducer casing and on a plate which serves as a pressure transmission medium and which is subjected to axial pre-stressing by the provision of an elastic tube spring, which is connected with the transducer casing and the pressure transmitting plate. The tube spring is arranged on a tubular diaphragm which seals the transducer element against the measuring pressure. A support member is also provided against which the tubular diaphragm rests in a radial direction, the support member being arranged between the diaphragm and the transducer element.

1 June 27, 1972 United States Patent Spescha 472,668 6/1969Switzerland..........................73/l33D [54] PRESSURE TRANSDUCER[72] Inventor: Gelll A. Speech, Winterthur, Switzerland Primary ExaminerDonald 0 Woodie] Attorney-Craig, Antonelli and Hill [73] Assignee:Kistler Instrumente AG, Winterthur, Switzerland [57] ABSTRACT A pressuretransducer employs a transducer element which is [22] Filed: June 19,1970 P supported on the transducer casing and on a plate which serves asa pressure transmission medium and which is subjected to axialpre-stressing by the provision of an elastic tube [30] ForeignApplication Prlorlty Data June 20, 1969 Switzerland 9553/69 spring,which is connected with the transducer casing and the pressuretransmitting plate. The tube spring is arranged on a 73/406, 73/398 R92/99 tubular diaphragm which seals the transducer element against 7/08the measuring pressure. A support member is also provided 73/393 AR 406,I33 D. against which the tubular diaphragm rests in a radial direction.

[$2] [1.5. CI. [51] Int. [58] Field the support member being arrangedbetween the diaphragm and the transducer element.

12 Claims, 6 Drawing Figures 56] References Cited FOREIGN PATENTS 0RAPPLICATION S 424,3l2 5/1967Switzerland..........................73/l33D PIATENTEDJUH 2 7 m2 SHEET 1OF 2 Inventor:

GELLI A. SPESCHA BY Cvoi RntZmU-L', Skewml: v

ATTORNEYS PATENTEIJJUI 21 m2 SHEET 2 OF 2 Inventor:

GELU A. SPESCHA BY (3on Qnronelli, StELOOI'l:

ATTORNEYS PRESSURE TRANSDUCER The application of mechanical/electricalpressure transducers has expanded considerably in the art of measurementand regulation. Thereby the mechanical values of the pressure of gasesor liquids whether static or dynamic can be measured at definite pointsby the fact that an electric signal corresponding to the pressure isevaluated by known methods. Both active and passive types of transducerscan be used. The conversion of a pressure into a force to be measured isusually effected by means of a resilient diaphragm in theforce-sensitive axis of the transducer. In this case the diaphragm isrigidly connected to the support body and acts directly through arigidly constructed pressure transmission plate to the force-transducingelement in that pressure forces acting on the diaphragm are convertedinto forces acting only along the sensitivity axis.

High flexure stresses are produced at each pressure change especiallyfor repeated pressure changes, as a result of the necessary axialflexure of the diaphragm. Attempts have been made to reduce thesestresses in that the diaphragm has been provided with one or morering-shaped depressions. Despite these measures the diaphragm is,however, the most strongly stressed component in normal commercialpressure transducers, especially if they are used for high pressureamplitudes and frequencies.

The working life of such pressure transducers is therefore very limitedand is generally determined by breakage of the diaphragm. In addition tothe danger of permanent breakage of the diaphragm, there is thepossibility of deformation of the diaphragm in operation, whereby thehearing point of the diaphragm is displaced and thus the conversionfactor pressure force is varied which acts directly on the sensitivityof the pressure transducer. These difficulties are mainly caused by thefact that the electricities of the transducer element and of thetransducer casing can be balanced one to the other only with difficulty.In particular, the problem also lies with the fact that radial and axialpressure forces act on the transducer casing beneath the sealing ring,while only axial forces should act on the transducer element.

The invention has for its object the provision of a pressure transducerin which a flat diaphragm sensitive to deformation and flexure isreplaced by a tube diaphragm elastic along the sensitivity axis of thetransducer. Such a diaphragm is only stressed by variations in thetension stressing along the diaphragm axis, whereby the danger ofdeformation, especially every flexure stress, is eliminated.

The invention thus refers to a pressure transducer w ith a transducerelement which is supported, on the one hand, on the transducer casingand, on the other hand, on a plate serving for pressure transmission andwhich, under the influence of an elastic tube spring connected with thetransducer casing and with the pressure-transmitting plate, is subjectedto axial pre-stressing. The invention comprises a tube spring arrangedon the tubular diaphragm which seals the transducer element against themeasuring pressure and in which the tubular diaphragm rests in theradial direction on a support member arranged between it and thetransducer element.

The invention will be further explained by way of example with referenceto the drawings wherein:

FIG. I shows a section through a commercial pressure transducer with anormal flat diaphragm,

FIG. 2 shows a portion of the same pressure transducer with an enlargedview of the deformation of the diaphragm part,

FIG. 3 shows a pressure transducer in section according to theinvention,

FIG. 4 shows a modification of a pressure transducer according to theinvention,

FIG. 5 shows a further arrangement of a pressure transducer according tothe invention; and

FIG. 6 shows an additional modification of a pressure transduceraccording to the invention.

The pressure transducer of usual commercial construction, shown insection in FIG. 1, consists ofa transducer casing l in which a pressuretransducer casing I which surrounds the transducer element 2 and screensit from radial pressure PI. The transducer element is covered by apressure transmitting plate 4 which is connected in turn with a flatdiaphragm 5 mechanically by welding or soldering. The surface pressureP2 acting on the flat diaphragm is received by the pressure transmittingplate 4 which is rigidly constructed, and is transmitted as a forcecomponent Z" to transducer element 2.

In order that the transducer element itself is subjected to no radialpressure P], the flat diaphragm must be firmly connected to theextension 3 of the support casing I. This is effected usually by weldingor soldering. In order that the pressure transducer can be mounted in acorresponding bore, it is provided with a mounting flange 6 which isisolated from the casing bore 8 by means of a sealing ring 7. Theclamping force necessary for this purpose is produced by a clamp ring 9.The transducer element 2, constructed in this case as a hollow cylinder,is mounted on the flat clamp surface II] in the transducer casing 1.Both active and also passive types can be provided as the transducerelement. By active" types of transducers are meant those which produce ameasuring signal by the measurement procedure itself, as for' example,piezoelectric transducers. By passive" types of transducers are meantthose which modify an impressed voltage as, for example, strain gaugestrip transducers. According to the element used the signal off-take II,sockets 12 are embedded in an insulating mass 13.

For normal installation, the surface pressure Pl compensates the frontalpressure P2. To reduce the bending tension in the flat diaphragm S, thelatter is provided with a depression I4 over the ring chamber betweenthe casing extension 3 and the pressure transmitting plate 4. With sucha depression M, a greater elasticity can be produced in the axialdirection.

The flexure of the flat diaphragm 5 is shown on an enlarged scale inFIG. 2. By the action of the frontal pressure P2 the diaphragm isdisplaced by the amounts since the casing extension 3 and the transducerelement 2 are not compressed to the same extent. The difficulty inbalancing the two elements for the same compression mass lies with thefact, as already stated, that not only the frontal pressure forces P2,but also the radial pressure forces PI act on the ring shaped extension3, while merely the resulting force acts on the transducer ele ment inthe axial direction. As a result of the difficulty in adapting these twoelements one to the other, there are always obtained axial deformationdifferences s which adversely affect the working life of the flatdiaphragm 5. Since, moreover, the bearing point 6 of the flat diaphragmcan be displaced as a result of this deformation, there is the dangerthat alterations in the sensitivity of the pressure transducer mayoccur.

The transducer according to the invention is shown in section in FIG. 3and comprises a casing 31 having an easily machined bearing face 40 onwhich the transducer element 32 is supported. The transducer element canbe of the active or passive type and in the example shown is in the formof a ring body with parallel end faces. A ring-shaped body 33 of thesame axial length as the element 32 is arranged around the transducerelement 32 in such manner that radial clearance l is left between thetwo of them. Thereby, the result is obtained that the transducer element32 cannot in any way be influenced by the ring body 33 by forces actingon its peripheral surface.

Advantageously, the transducer element 32 and the ring body 33 aresimultaneously machined flat on their end faces. A tubular diaphragm 34,resilient along its longitudinal axis, rests on the ring body 33 and maybe formed in one piece with the pressure transmitting plate 35 and themounting flange 36. The ring-shaped body 33, thus operates as a supportmember for the tubular diaphragm. The bell-shaped unit is subject to apredetermined mechanical prestress and the flanged part 36 is rigidlyconnected by welding or a similar joint 37 to the transducer casing 31.In this way springing action between the transducer element, thepressure transmitting place and the transducer casing is largelyavoided.

By the omission of the flat diaphragm, the pressure transmitting plate35 can be arranged directly as a closure means, relative to the pressuremedium, and hence, according to the purpose of application, a suitablematerial is chosen. Generally, care is taken that this plate has assmall a mass as possible. that is to say, it is produced from materialof low specific weight n the other hand, the plate must be as rigid aspossible in order that the surface pressure P2 acting on it shall causethe minimum possible deflection on the bearing faces 38 of thetransducer element and that further, the plate shall transmit to thetransducer element a force Z which is directed along the axis asaccurately as possible.

The force 2 is transmitted partially to the transducer element andpartially to the ring body, according to the elasticity and dimensionrelations of the transducer element 32 and the ring body 33. By theelastic or resilient formation of the ring body, the result can beobtained that almost the whole force Z is transmitted to the transducerelement. The main function of the ring body 33 is to absorb the radialforces Pl received by the tubular diaphragm 34 without transmitting themto the transducer element 32. The construction should, however, be suchthat no disturbing transverse deflections appear by reason of theseradial forces, which could influence the measurement signal of thetransducer element.

The elastic yield produced by the frontal forces P2 is absorbed both bythe transducer element and by the ring body and is compensated by theelastic tube spring. The changes in length of the tube spring 34, as aresult of varying forces produced in this way, produce no bending forcesthereon, but only a variation in the predetermined longitudinal tension.The tube spring can, therefore, absorb far greater changes in shapesince the whole length of the elastically constructed part can beutilized for this purpose. The stress is thus converted from a bendingstress, as is present in the diaphragm sealing according to FIG. 1, intoa pulsating tensional stressing in which, as a result of the far greaterdeflection lengths, a much smaller specific stressing is obtained thanwith a flat diaphragm designed for similar conditions in which there isonly a very limited scope for a relatively large change in shape. Theworking life of such a tubular diaphragm is therefore many times higherthan for a flat diaphragm. Furthermore, the rigid transmitting plate 35has also the advantage that it produces no changes in the bearing pointsduring deflection and thus the response does not vary with the pressure.

FIG. 4 shows a modification of the construction according to theinvention in which the ring body 43, constructed as a pressure ring ismade substantially more elastic in the axial direction by means ofdepressions 46. In this way the elasticity is less varied in the radialdirection. This feature has the further advantage that any movementproduced by the deflection 5 between the tube diaphragm 44 and thepressure ring 43, is absorbed by deflection of the individual rings 45.In this way, all possible mechanical friction between the elastictubular diaphragm and the pressure ring is avoided. It is, however, alsoimmediately possible to construct the pressure ring 43 from a pluralityof discs (see FIG. 6), instead of in one piece, which are provided withsuitably chosen intermediate rings. In this way the longitudinalelasticity of the pressure ring element can be adapted to a large extentto local requirements.

A further modification of a pressure transducer according to theinvention is shown in FIG. 5 in which the ring body 51 is formed in onepiece with the transducer body 52, in the same way as the transducer inFIG. 1. Depressions 54 are also provided to further increase thelongitudinal elasticity but espe cially to avoid friction relativelywith the tubular diaphragm 55. The ring body 51 can extend up to thepressure transmitting plate 56, whereby it participates in the forcepath due to the frontal forces P2. It can, however, also be shorter sothat a clearance a is left from the pressure transmitting place 56,whereby it must then absorb only the radial forces caused by thepressure forces P]. This solution has the great advantage that the massof the ring body 51 only acts to an insignificant extent for increasingthe acceleration sensitivity of the transducer.

In this case, a separate construction of the pressure transmitting plate56 and the tube diaphragm 55 is shown as an example, wherein the latteris provided with a ring-shaped rim 57. In this way it is possible toform the pressure transmitting plate 56, for example, of a ceramic orglass-like material, while the tube diaphragm 55 can be made, forexample, from metal or also from a glass or ceramic like material. FIG.6 illustrates an additional variation of the instant pressure transducerwherein, instead of the single ring body, a plurality of discs 58,separated by intermediate rings or spacers 59 are provided. Aspreviously noted, the pressure ring element is thus given additionallongitudinal elasticity.

The problem of frictional effects between the tube diaphragm 34 and thering body 33 can also be eliminated by the fact that the operative axiallength of the tube diaphragm 34 is made the same as that of the ringbody 33. In this way the transducer element 32, the ring body 33 and thetube diaphragm 34 execute equal elastic movements so that no frictioncan be produced. In applications where, as far as possible, equal axialheat expansion and elasticity of piezoelectric arrangement of thecrystals and of the ring body are required, it is desirable to form thelatter also of the piezoelectric crystal material.

The invention so far described thus represents a considerable advance inthe field of pressure transducers. The replacement of the usual flatdiaphragm by an elastic tube diaphragm, which can be supported in theradial direction on a ring body, provides the possibility that thepressure transducer element can be subjected to a flat parallel loadingwherein also the rigid construction of the pressure transmitting plateproduces no change in the pressure bearing point even for deflectionunder high pressure action. It offers, however, also the possi bility offulfilling two quite different requirements of forming the pressuretransmitting plate and the elastic tube diaphragm according to desiredrequirements. Thus, for example, the pressure transmitting plate can beconstructed of a very rigid plate which is as light as possible and withthe smallest possible coefficient of expansion and the smallest possibleconductivity, while the elastic tube diaphragm can be produced of steelwith good elasticity properties. The connection of the two ele ments canbe obtained both by a thick collar part or, according to recent methods,by means of hard soldering. Also, the pressure transmitting plate can beproduced, for example, from a ceramic material and the elastic tubediaphragm from an elastic nickel steel. It is, however, also possible toproduce the whole unit, for example, from moulded quartz glass. Pressuretransducers according to the invention are constructed in the same wayas commercial types as regards the installation arrangements and thesignal take-off means.

The invention being thus described, it will be obvious that the same maybe varied in many ways. Such variations are not to be regarded as adeparture from the spirit and scope of the invention, and all suchmodifications as would be apparent to one skilled in the art areintended to be included.

What I claim is:

1. A pressure transducer comprising a casing, a plate serving forpressure transmission, a transducer element supported by said casing andsaid plate, an elastic tubular diaphragm connected to said casing andsaid plate and sealing said transducer element against the measuredpressure, said elastic tubular diaphragm subjecting said transducerelement to axial prestressing, and a support member positioned betweensaid elastic tubular diaphragm and said transducer element.

2. A pressure transducer according to claim I, characterized in thatsaid support member is a ring body which surrounds said transducerelement in the manner of a ring.

3. A pressure transducer according to claim 2, characterized in thatsaid ring body is clamped between said pressure transmitting plate andsaid transducer casing and is under axial pre-stressing pressureproduced by said tubular diaphragm.

4. A pressure transducer according to claim I, characterized in thatsaid support member is constructed to prevent radial stresses fromacting on said transducer element.

5. A pressure transducer according to claim 1, characterized in thatsaid elastic tubular diaphragm, said pressure transmitting plate and itsmounting flange are formed in one piece.

6. A pressure transducer according to claim 1, characterized in that thering body included projecting elements distributed around its outerperipheral surface, on which said tubular diaphragm bears.

7. A pressure transducer according to claim 6, characterized in that theprojections are formed as ribs extending in the peripheral direction ofthe ring body.

8. A pressure transducer according to claim 1, characterized in that thepressure transmitting plate consists of a rigid material having a lowerheat conductivity and a lower specific weight than the casing.

9. A pressure transducer according to claim I, characterized in that thetransducer element consists of a piezoelectric element and that thesupport member is formed of a piezoelectric crystal material 10. Apressure transducer according to claim 2, characterized in that the ringbody consists of individual discs placed one on the other and betweenwhich spacing means can be insertedi 11. A pressure transducer accordingto claim L characterized in that the effective axial length of thetubular diaphragm is the same as the axial length of the ring body.

12. A pressure transducer according to claim I, characterized in thatthe support member and the tube diaphragm are formed in one piece.

1. A pressure transducer comprising a casing, a plate serving forpressure transmission, a transducer element supported by said casing andsaid plate, an elastic tubular diaphragm connected to said casing andsaid plate and sealing said transducer element against the measuredpressure, said elastic tubular diaphragm subjecting said transducerelement to axial prestressing, and a support member positioned betweensaid elastic tubular diaphragm and said transducer element.
 2. Apressure transducer according to claim 1, characterized in that saidsupport member is a ring body which surrounds said transducer element inthe manner of a ring.
 3. A pressure transducer according to claim 2,characterized in that said ring body is clamped between said pressuretransmitting plate and said transducer casing and is under axialpre-stressing pressure produced by said tubular diaphragm.
 4. A pressuretransducer according to claim 1, characterized in that said supportmember is constructed to prevent radial stresses from acting on saidtransducer element.
 5. A pressure transducer according to claim 1,characterized in that said elastic tubular diaphragm, said pressuretransmitting plate and its mounting flange are formed in one piece.
 6. Apressure transducer according to claim 1, characterized in that the ringbody included projecting elements distributed around its outerperipheral surface, on which said tubular diaphragm bears.
 7. A pressuretransducer according to claim 6, characterized in that the projectionsare formed as ribs extending in the peripheral direction of the ringbody.
 8. A pressure transducer according to claim 1, characterized inthat the pressure transmitting plate consists of a rigid material havinga lower heat conductivity and a lower specific weight than the casing.9. A pressure transducer according to claim 1, characterized in that thetransducer element consists of a piezoelectric element and that thesupport member is formed of a piezoelectric crystal material.
 10. Apressure transducer according to claim 2, characterized in that the ringbody consists of individual discs placed one on the other and betweenwhich spacing means can be inserted.
 11. A pressure transducer accordingto claim 1, characterized in that the effective axial length of thetubular diaphragm is the same as the axial length of the ring body. 12.A pressure transducer according to claim 1, characterized in that thesupport member and the tube diaphragm are formed in one piece.